This invention relates to a low-noise level internal combustion engine, more particularly to a reciprocating piston internal combustion engine provided with means for suppressing engine noise at a considerable low level.
In conventional reciprocating piston engines, the journal of a crankshaft is rotatably supported by main bearings whose upper half parts are carried by a cylinder block. The lower half parts of the main bearings are carried by bearing caps which are secured to the bottom portion of the cylinder block by means of bolts. Additionally, the cylinder block is integrally provided at its bottom portion with a skirt portion which bulge out to spacedly cover the rotating crankshaft. An oil pan for reserving an engine oil is sealingly secured to the skirt portion.
With such conventional engines, the oil pan is constructed and arranged to serve only as a container for the engine oil. Therefore, the oil pan does not contribute to increase in flexural rigidity of the engine, and additionally serves as means through which the explosion sounds in engine cylinders and the bearing sounds are radiated out of the engine. Additionally, the bulged skirt portion of the cylinder block is liable to vibrate and accordingly serves as a source of engine noises.